Light emitting diode array current power supply

ABSTRACT

A power supply circuit for an LED print head has a reference current source connected to a printer system reference voltage for providing a second reference voltage for an individual, integrated circuit chip. The reference current source is controlled by an operational amplifier, the output of which comprises the second reference voltage. A plurality of output driver FETs are biased by the reference voltage. Each output driver FET provides current to an associated LED responsive to a data signal. A plurality of control FETs are connected in parallel with each other and in series with each output driver FET for varying the LED output current responsive to programming data signals. A second group of control FETs is interposed in the feedback loop of the operational amplifier for varying the second reference voltage responsive to a different set of programming data signals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to light emitting diode arraycurrent power supplies and more particularly to such supplies which usean operational amplifier to regulate a constant current source.

2. Description of the Related Art

It has become desirable to employ non-impact xerographic-type printersfor text and graphics. In such a printer, an electrostatic charge isformed on a photoreceptive surface of a moving drum or belt and selectedareas of the surface are discharged by exposure to light. A printingtoner is applied to the drum and adheres to the areas having anelectrostatic charge and does not adhere to the discharged areas. Thetoner is then transferred to a sheet of plain paper and is heat-fused tothe paper. By controlling the areas illuminated and the areas notilluminated, characters, lines and other images may be produced on thepaper.

One type of non-impact printer employs an array of light emitting diodes(commonly referred to herein as LEDs) for exposing the photoreceptorsurface. A row, or two closely spaced rows, of minute LEDs arepositioned near an elongated lens so that their images are arrayedacross the surface to be illuminated. As the surface moves past the lineof LEDs, they are selectively activated to either emit light or not,thereby exposing or not exposing, the photoreceptive surface in apattern corresponding to the LEDs activated.

To form good images in an LED printer, it is desirable that all of thelight emitting diodes produce the same light output at the image planewhen activated. This assures a uniform quality image all the way acrossa paper. The light output at the image plane depends on a number offactors including current, temperature, lens transmittance factors andprocessing parameters for forming the LED which may affect its lightoutput as a function of current.

Light emitting diodes for print heads are formed on wafers of galliumarsenide or the like, suitably doped to conduct current and emit light.Long arrays of LEDs are formed on a wafer which is cut into separateddice, each having an array of LEDs. A row of such dice are assembledend-to-end to form a print head array. The light output of the LEDs on agiven die are usually reasonably uniform, however, there may bevariations from die to die as processing parameters differ between dice.There is some variation within dice from an individual wafer and greatervariation from wafer to wafer.

The LEDs are driven by power supplies on integrated circuit chips. Thecurrent output of these chips may also vary depending on processingparameters in making these chips. Such variations may compound thevariations in light output.

A parameter that is partly LED power supply dependent is the rise timefor current flow. This is significant, since the exposure of thephotoreceptive surface is a function of both intensity and illuminationtime. In an LED print head, there may be a few thousand LEDs across thewidth of the photoreceptive surface. The current in each LED may also beaffected by the number of LEDs enabled at any time. Thus, there may be arelatively high current and concomitant higher light intensity or totalexposure when a few LEDs are enabled, as compared with the current andlight output when a very large number of LEDs are enabled.

Prior U.S. Pat. No. 4,864,216 to Kalata et al., which is incorporatedherein by reference, provides a power supply for an LED print head inwhich a chip reference voltage biases a plurality of output driver FETsto provide substantially the same preselected current level to an LEDassociated with each output driver FET. The current is switched by adata signal applied to a data FET in series with each driver FET andLED. The Kalata et al. power supply represents a significant advance inthe art in that it assures uniform light output across the array and alight output substantially independent of differences in the number ofLEDs enabled.

It can be seen that in an ideal system, the current signal applied toeach diode is a square wave of equal duration and magnitude. Such asignal generates uniform exposure of the charged surface, assuming equallight output for each LED at the image plane. The Kalata et al. powersupply, while producing light substantially uniformly, tends to generatea current square wave with overshoot on the leading edge. This is aresult of capacitive coupling between the gate and drain of the outputdriver FETs. When the data FET in a selected leg of the power supplyswitches on, the voltage on the drain of the output driver drops andtends to drag down the chip reference voltage applied to the gate of theoutput driver FET. This tends to turn on the FET harder thus providingmore current until the capacitive coupling discharges. When suchdischarge occurs, the chip reference voltage returns to the selectedlevel thereby dropping the current level through the output driver FETto the desired level.

As noted above, variations from one LED to another can cause variationsin light output given the same current through each LED. Similarly,variations from chip to chip can cause differences in the average totalchip light output given the same reference voltage on each chip. TheKalata et al. power supply suggests a variable resistor to change thechip reference voltage.

It would thus be desirable to compensate for variations from chip tochip and from LED to LED within a single chip. It would be advantageousto achieve such compensation with data programming signals.

It would also be desirable to provide a current through each LED whichis substantially a square wave, i.e., without any, or at least withoutany consequential, overshoot or undershoot.

It would also be desirable to provide a light emitting diode arraycurrent power supply which can be powered by a lower current supplyvoltage than such prior art power supplies.

BRIEF SUMMARY OF THE INVENTION

In one aspect, the present invention comprises a reference FET and aplurality of output driver FETs, each of which has an output connectionfor an LED. The gates of all of the FETs are connected to a lowimpedance voltage source. A constant current is passed through thereference FET. Current flows from a selected output driver into itsassociated LED responsive to a data signal.

In another aspect of the invention, means are provided for varying thechip reference voltage responsive to programming data signals.

In still another aspect, the invention includes a plurality of controlFETs in parallel with one another and in series with the outputconnection. The current applied to each LED is a function of the chipreference voltage and the bias condition of the control FETs.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will beappreciated as the same becomes better understood by reference to thefollowing detailed description when considered in connection with theaccompanying drawings wherein:

FIG. 1 illustrates in block form a plurality of integrated circuit chipsfor an exemplary power supply; and

FIG. 2 illustrates a power supply circuit, constructed in accordancewith the present invention, for each such integrated circuit chip.

DETAILED DESCRIPTION

An exemplary LED print head has a row of 228 LED dice placed end-to-endto stretch across the width of a photoreceptive surface. Each die has 64LEDs along its length. The LED dice are made in large numbers on agallium arsenide wafer, which is then cut up to form the individualdice. It is found that there are variations in LED light output as afunction of current from wafer to wafer, due to differences inprocessing variables. Generally speaking, all of the LEDs on a die arequite similar to each other in this characteristic. Dice from differentwafers may differ appreciably in light output as a function of current.Dice from various portions of a wafer may fall somewhere in between. Thelight output from the LEDs on a large number of LED dice tends to have amore or less Gaussian distribution around a desired light output.

To minimize this source of non-uniformity in LEDs used in practice ofthis invention, the light output for each LED die is measured and thedice are sorted into collections or "bins" so that all of the dice in agiven bin have a variation in light output much smaller than thevariation occurring in the total population of LED dice. When a givenprint head is assembled, all of the dice are taken from a single bin sothat the intrinsic light output as a function of current issubstantially the same for all of the LEDs on that print head. What isnext needed is a power supply which delivers a substantially square wavecurrent pulse to each LED in the array and which further includes meansfor varying the current from chip to chip as well as from LED to LED ona selected chip responsive to programming data signals.

Power is supplied to the LEDs from integrated circuit chips mounted inclose proximity to the LED dice. In an exemplary embodiment, anintegrated circuit chip is located next to an LED die and containscircuits for delivering current to the LEDs on the LED die. Such a chipmay include a variety of other print head operational circuits which donot form a part of this invention. For example, data signal multiplexingcircuits may be included on the chip.

Thus, as indicated schematically in FIG. 1, there may be a row ofintegrated circuit chips IC₁, IC₂ . . . IC_(n) mounted near a row of LEDdice (not shown). A power supply voltage V_(C), is applied to each ofthe integrated circuit chips. A system reference voltage, V_(R) is alsoapplied to each chip. The magnitude of the system reference voltage canbe set so that the light output from a given LED print head achieves adesired average level. For example, if the intrinsic light output fromthe set of LEDs on the print head is lower than the mean of the Gaussiandistribution of light outputs, the reference voltage for that print headmight be a higher value than for a print head having an intrinsic lightoutput closer to the mean. Thus, by varying the system referencevoltage, the light output of all of the LEDs in the array can be raisedor lowered, as desired, in synchronism.

In addition to variations that may occur due to processing variables ofthe LED dice, there may be variations in the properties of theintegrated circuit chips supplying power to the LEDs. The LED printheads are analog devices and such variations may be more significantthan in digital circuits.

To partially compensate for such possible variations, each integratedcircuit chip is provided with a reference resistor, R₁, R₂ . . . R_(n),the value of which may be selected for assuring that all of theintegrated circuit and LED sets on a given print head producesubstantially the same light output. As will be described, the presentinvention is implemented in a circuit which also permits control of theaverage LED light output on a selected chip as well as control ofcurrent variations from LED to LED on a selected chip, both responsiveto programming data signals. Typical resistance values for the referenceresistors lie in the range from 1,000 to 2,000 ohms.

A portion of the circuits on a representative integrated circuit chipare illustrated in FIG. 2. In this drawing, a phantom line indicates theportion of the circuit lying on the chip as distinguished fromcomponents such as a reference resistor R_(r) which, as just mentioned,is preferably located off of the integrated chip. Contact pads formaking connections to the chip are omitted, as are many other details ofcircuits on such a chip which are not material to an understanding ofthis invention.

The reference voltage, V_(R), is applied to the inverting input of aconventional operational amplifier 10 of a reference current cell oneach chip. Operational amplifier 10 is also referred to herein as alow-impedance voltage source. The operational amplifier is formed by thesame processes employed for the balance of the circuitry on theintegrated circuit chips.

The operational amplifier output is connected to the gates of aplurality of FETs 12-20 in a reference leg, indicated generally at 22,of the circuit. FETs 12-20 are collectively referred to herein as areference FET. The output of operational amplifier 10 provides a systemreference voltage V_(r) which is applied to the gates of each of FETs12-20. The drain of each of FETs 12-20 is connected to the source of acorresponding p-channel FET 24-32. FETs 24-32 are referred to herein ascontrol FETs. The gate of FET 24 is tied to a preselected biasingvoltage, V_(b), which maintains it in a conducting condition. The gatesof FETs 26-32 are connected to a source of programming data signalswhich place each FET in either a conducting or nonconducting conditiondepending upon whether the programming data signal for the FET is in alow or high state, respectively.

In reference leg 22, amplifier 10 controls the gates of FETs 12-20 andincreases or decreases V_(r) until the voltage at the reference resistormatches the external reference voltage V_(R) at the inverting input atthe amplifier. Thus, current equal to V_(R) /R_(r) flows upwardly inreference leg 22 for all operating conditions of the circuit.

FET 12 and FET 24 each have the same width as does each of the otherreference-control FET pairs in reference leg 22. However, each of FETs12-20 and its corresponding control FET 24-32, respectively, has achannel width different from each of the other reference-control FETpairs. FETs 12, 24 have the widest channel width comprising 70% of thesum of channel widths of all FET pairs in leg 22. In the presentembodiment, the ratio of the channel widths of FETs 14-20 is 1 to 2 to 4to 8, respectively. By selecting different combinations of FETs 26-32,the channel width of the reference FET can be varied from 70% to 100% ofthe total channel width of FETs 12-20 in 2% increments. It can thus beseen that programming data signals applied to the gates of FETs 26-32may be used to vary V_(r) because amplifier 10 drives to maintain V_(R)across R_(r). Stated another way, the programming data signals appliedto the gates of FETs 26-32 change the resistance of the feedback loop ofamplifier 10 thereby changing the output voltage, V_(r). Each of FETs12-20 have the same length as do FETs 24-32. The length of the channelsof FETs 24-32 are relatively short to facilitate their switchingfunction.

Indicated generally at 34, 36 are output legs of the circuit with thecircuit comprising a total of n output legs. In output leg 34, FETs38-46 are collectively referred to herein as an output driver FET. Thegate of each of FETs 38-46 is tied to the chip reference voltage V_(r).The drain of each of FETs 38-46 is tied to the source of a correspondingp-channel FET 48-56, respectively. The drain of each of FETs 48-56 isreferred to herein as an IC chip output and is connected to acommercially available LED 58 which is located off the chip.

The gate of FET 48 is tied to a source of data signals D₁ which has twoconditions: one which permits FET 48 to conduct and the other which cutsit off. The data signal is provided from a computer output for causingprinting as described above in a pattern determined by the computeroutput.

The gates of each of FETs 50-56 is connected to a source of programmingdata signals which are generated as a function of data signal D₁ andanother signal which determines whether the FET is conducting or cut offwhen FET 48 conducts. FETs 38-46 have the same length as FETs 12-20. Asin reference leg 22, the FET pairs in each of the output legs have thesame channel width. Of the total channel widths of FETs 38-46, FET 38has 47.5% of the total width. Each of FETs 40-46 have different smallerchannel widths. The ratio of the channel widths of FETs 40-46 is 1 to 2to 4 to 8, respectively. By selecting different combinations of FETs40-46 to switch in ganged relation with FET 38, responsive to theaforementioned programming data signals, the total channel width of FETs38-46 can be made to vary from 47.5% of the entire width to 100% thereof(when each of FETs 40-46 conduct in synchrony with FET 38) in 3.5%increments.

It can thus be seen that the current flowing through LED 58, and thusthe intensity of light emitting therefrom, is controllable responsive toprogramming data signals applied to FETs 50-56.

Each of the other output legs, like leg 36, can be similarly controlledby the same, or different, programming data signals applied to controlFETs 50-56 in output leg 34.

It can thus be seen that V_(r) is selectable in accordance with signalsapplied to the gates of FETs 26-32. These data programming signals areselected to be either a high or a low value dependent upon the averagelight intensity desired for each of the LEDs driven by the chip. Itshould be noted that as the total gate width of FETs 12-20 decreases byswitching off selected ones of FETs 26-32, V_(r) decreases therebydriving FETs 38-46 into a greater conducting condition. Such actionincreases the current and thus the light output of LED 58.

Similarly, the current from LED to LED on the chip may be changed byselecting different combinations of FETs 50-56 which switch in synchronywith FET 48. Variations in light output may thus be made by changingwhich of FETs 50-56 are so switched in a given output leg of thecircuit.

It should be noted that the output impedance of amplifier 10, as is thecase with a conventional low-gain operational amplifier, is low relativeto the output impedance of a single FET. As data FETs, like FETs 48-56switch in various output legs of the chip, the op amp drive voltage doesnot sag as a result of a capacitive coupling between the sources of thedata FETs and the gates of the reference FETs, like FETs 38-46. Theproblem of current overshoot in the current square wave generated by thechip outputs is thus overcome.

Another advantage obtained by the present invention relates to the useof a relatively low power supply voltage V_(C). In Kalata et al., thegate of the reference FET is connected to its drain. Another FET isconnected from that node to the reference resistor. Thus the requiredpower supply voltage is the sum of the voltages across those threecomponents. With respect to the reference FET, the critical voltage isthe voltage from its gate to its source. In the present invention, thegate to source voltage of reference FET 12 is determined by the outputof the operational amplifier. It is thus not constrained to be less thanV_(C) minus the voltage on resistor 24. Rather it can be equal to all ofV_(C). Considered another way, this means that V_(C) in the presentinvention can be less than the V_(C) in Kalata et al. by the value ofthe voltage across the reference resistor, namely V_(R).

Another advantage of the present invention is improved tracking ofcurrent from the input reference to the output pad. The system referencevoltage V_(R) can be chosen to match that of a typical LED while beingdriven with a nominal current. In that case the reference FETs match theoutput driver FETs with respect to drain voltage, gate voltage, sourcevoltage, and bulk voltage. Since these FETs also are matched in gatelength, their electrical conditions are fully matched. Their draincurrent per gate width thus matches. The current delivered to each LEDtherefore is a value which is quite stable and easily predicted basedupon the total current entering the reference FETs, and the ratio of thetotal enabled width of the corresponding output driver to the totalenabled width of the reference FETs.

Having illustrated and described the principles of my invention in apreferred embodiment thereof, it should be readily apparent to thoseskilled in the art that the invention can be modified in arrangement anddetail without departing from such principles. I claim all modificationscoming within the spirit and scope of the accompanying claims.

I claim:
 1. A power supply for a light emitting diode print headcomprising; a reference FET;a plurality of output driver FETs, eachhaving an output for connection to a single LED in the print head; a lowimpedance voltage source; means for interconnecting the gates of all theFETs to said low impedance voltage source; means for passing a constantcurrent through the reference FET; means for enabling current flow fromeach output driver FET to tis respective LED in response to a datasignal, said enabling means comprising: a control FET connected inseries with each output driver FET; and means for biasing said controlFETs in response to programming data signals thereby controlling thecurrent level through the LED.
 2. The power supply of claim 1 whereinsaid low impedance voltage source comprises an operational amplifier. 3.The power supply of claim 2 wherein said means for passing a constantcurrent comprises a current source regulated by said operationalamplifier and a reference resistor for setting the current.
 4. The powersupply of claim 1 wherein said means for enabling current flow comprisesa data FET in series with each output driver FET and means for applyinga data signal to the gate of the data FET.
 5. A power supply for a lightemitting diode print head comprising:a plurality of reference FETs; aplurality of output driver FETS, each having an output for connection toan LED; a low impedance voltage source; means for interconnecting thegates of all the FETs to said low impedance voltage source; means forpassing a constant current through each reference FET; means forenabling current flow from each output driver FET to its respective LEDin response to a data signal; means for varying the voltage produced bysaid source, said varying means comprising: a control FET connected inseries with each reference FET; and means for biasing said control FETsin response to programming data signals thereby controlling the totalcurrent level through the reference FETs.
 6. The power supply of claim 5wherein said means for enabling current flow from each output driver FETto its respective LED in response to a data signal further comprises:aplurality of output driver FETs, each having an output for connection toa single LED in the print head; a control FET connected in series witheach output driver FET; and means for biasing said control FETs inresponse to programming data signals thereby controlling the currentlevel through the LED.
 7. A method for providing a substantially squarewave current to a plurality of LEDs comprising the steps of:generating avoltage having a relatively low drive impedance; passing a constantcurrent through a reference FET; providing the voltage to the gate ofthe reference FET and to a plurality of output drive FETs, each of whichhas an output connected to a single LED in the print head; generating adata signal; and enabling current flow from selected output driver FETsresponsive to the data signal, said enabling step comprising the step ofenabling or preventing current flow through selected ones of a pluralityof control FETs, each of which is connected in series with a differentoutput driver FET.
 8. The method of claim 7 wherein said method furthercomprises the step of using the voltage to regulate a current source. 9.The method of claim 8 wherein the step of passing a constant currentthrough a reference FET comprises the step of passing current generatedby the current source through the reference FET.
 10. A method ofproviding a substantially square wave current to a plurality of LEDscomprising the steps of:generating a voltage having a relatively lowdrive impedance; passing a constant current through a plurality ofreference FETs; providing the voltage to the gates of the reference FETsand to a plurality of output driver FETs, each of which has an outputconnected to an LED; generating a data signal; enabling current flowfrom selected output driver FETs responsive to the data signal; andvarying the low-drive-impedance voltage, said varying step comprisingthe step of enabling or preventing current flow through selected ones ofa plurality of control FETs, each of which is in series with adifference reference FET.
 11. The method of claim 10 wherein the step ofenabling current flow from selected output driver FETs responsive to thedata signal comprises the step of enabling or preventing current flowthrough selected ones of a plurality of control FETs, each of which isin series with a different output driver FET.
 12. A power supply for alight emitting diode print head comprising:a plurality of integratedcircuit chips, each chip having a plurality of outputs for supplyingcurrent to each of a plurality of respective LEDs; means for applying asystem reference voltage to each of the integrated circuit chips; meansfor generating a chip reference voltage for each of the plurality ofchips in response to the system reference voltage, said generating meanscomprising a plurality of control FETs in a feedback loop thereof; meansfor applying a current to each LED output which is a function of therespective chip reference voltage and the presence or absence of a datasignal for such LED output; and means for varying the chip referencevoltage of each chip responsive to programming data signals appliedthereto.
 13. The power supply of claim 12 wherein said means forgenerating a chip reference voltage comprises an amplifier having aplurality of control FETs in a feedback loop thereof.
 14. The powersupply of claim 12 wherein said means for varying the chip referencevoltage comprises means for biasing said control FETs to differentconducting conditions thereby varying the amplifier output voltage. 15.The power supply of claim 14 wherein said means for biasing said controlFE different conducting conditions thereby varying the amplifier outputvoltage comprises means for enabling or preventing current flow throughselected ones of said control FETs responsive to data signals.
 16. Thepower supply of claim 12 wherein said means for applying a current toeach LED output comprises an output driver FET and wherein said supplyfurther includes a plurality of control FETs connected in parallel withone another and in series with each output driver FET and means forbiasing said control FETs to different conducting conditions.
 17. Thepower supply of claim 16 wherein said means for biasing said controlFETs to different conducting conditions comprises means for enabling orpreventing current flow through selected ones of said control FETsresponsive to programming data signals.